A composite material is composed of one or more reinforcing materials embedded in a matrix material. Composite materials having high degrees of utility typically exhibit mechanical, or other properties, superior to those of the individual materials from which the composite was formed. A common example of a composite material is fiberglass. Fiberglass is produced by imbedding glass fibers, which are the reinforcing material, in a resin, which constitutes the matrix material. Composites utilizing organic polymeric matrix materials are well known and have been widely utilized. However, the properties of such composites, although sometimes exceptional, do have limitations with respect to strength and temperature compatibility. Other composites have been developed that utilize metals as the matrix material. Such metal matrix composites can exhibit properties, such as temperature resistance, superior to those of organic polymeric matrix composites.
Generally, composite materials constitute a class of materials that provide for design flexibility by allowing their properties to be tailored within limitations according to the specific requirements for different applications. For example, metal matrix composites, such as aluminum matrix composites may be used for a variety of structural and non-structural applications, including applications for electronics, automotive and aerospace industries.
Composite materials are generally classified on the basis of the shape and size of the reinforcements. One type of composite material, a unidirectionally aligned fiber composite, contains fibers of a critical length that are arranged in parallel and are aligned along the length of the composite. FIG. 1 shows a representation of a magnified view of such a unidirectionally aligned fiber composite (10) consisting of a matrix metal (20) which is reinforced with ceramic fibers (30). Another type of composite material is a discontinuous fiber composite. In such a composite, relatively short lengths of fiber reinforcement, sometimes referred to as whiskers, are arranged randomly in the matrix material. FIG. 2 shows a representation of a magnified view of a discontinuously reinforced metal matrix composite (40). The reinforcing material used in this composite is a discontinuous ceramic fiber (50) and the matrix material (60) is a metal. Discontinuously reinforced composites may also be prepared using a particulate reinforcement dispersed in a matrix material. A magnified view of such a discontinuously reinforced particulate composite (70) is represented in FIG. 3. In this representation, the particulate reinforcements (80) are dispersed in the metal matrix (90).
The properties of composite materials are generally influenced by the properties of the matrix material as well as by the properties, including type, shape, size, and volume fraction, of the reinforcing material. The main strengthening mechanism of unidirectionally aligned fiber composites is based on load transfer from the matrix to the fibers. Therefore the load is mainly carried by the fibers. The highest levels of strength and stiffness are typically attained using continuous, strong, fibers aligned in the direction of loading, such as is provided by continuous fiber composites, as the strong fibers carry the majority of the load. Although unidirectionally aligned fiber composites, including continuous fiber composites, have superior strength in the direction of the fibers, their applications are often limited by their high costs of production, the problems associated with their processing, and their inferior transverse properties.
Generally, discontinuously reinforced composites are weaker than are unidirectionally aligned fiber composites along the fiber direction. However, discontinuously reinforced matrix composites are attractive for reasons such as their low cost and increased flexibility in processing. Additionally, such composites have isotropic mechanical properties. This isotropic nature can result in discontinuously reinforced composites being preferable to unidirectionally aligned fiber composites in applications requiring composite strength in more than one direction.
Discontinuously reinforced particulate composites can encompass a very wide range of reinforcing particulate sizes. For example, one type of discontinuously reinforced particulate composite is dispersion strengthened metals. Dispersion strengthened metals are reinforced with submicron sized hard particles that directly inhibit dislocation motion in the matrix through the Orowan mechanism. Generally, the required volume fraction of the particulate phase in dispersion strengthened metals is relatively small. Such dispersion strengthened metals may be used, for example, for elevated temperature applications. However, the preparation of such dispersion strengthened materials typically requires extensive and expensive processing.
A second type of discontinuously reinforced particulate composite utilizes particulates of about 1 micron to 50 micron size. In this particulate reinforcement size range stiffness and strength enhancements can occur. Such composites are typically less difficult to produce than the first type.
A third type of discontinuously reinforced particulate composite utilizes even coarser particulates. The size of the particulates exhibited in these types of discontinuously reinforced particulate composites is in the range of about 50 to 250 μm. This third type of particulate composites typically provides greater production flexibility and ease of production. Applications for which such composites are typically useful are those requiring wear resistance.
There are various factors that influence the mechanical behavior of particulate composites. These factors can include the nature and type of the particulate phase (strength and deformability), particle size, volume fraction, shape of particles (aspect ratio), coefficient of thermal expansion (CTE) of the matrix and the particulate material, bond strength between the matrix and the particulate material, and overall matrix characteristics.
With respect to the three types of composites previously discussed, the strengthening mechanism of the first type of composites is primarily dispersion strengthening. The strengthening mechanism of those composites of the second and third types generally involves several components, such as matrix strengthening, thermal residual stresses through coefficient of thermal expansion (CTE) mismatch, and load transfer from the matrix to the particles. The aspect ratio of the particles is an important factor that influences the load transfer from the matrix to the particles. The extent of strengthening in these particulate composites increases as the particle size decreases and also with the increase in the amount of particulate phase.
Load sharing by the particles occurs in a discontinuously reinforced matrix. Typically, however, particles share a smaller amount of the load than do fibers. Matrix strengthening also contributes to the overall strength of discontinuously reinforced metal matrix composites. In metal matrixes, the reinforcing effects of particulates include various other strengthening mechanisms. For example, the particulates may constrain plastic deformation of the metal matrix.
For example, particulate silicon carbide (SiC) is commonly used as a reinforcing material in discontinuously reinforced metal matrix composite materials. In particular, composites composed of aluminum matrices with silicon carbide particulates, as the reinforcing material, are commonly used. However, the load sharing by the silicon carbide particles is limited by the inherently weaker bond exhibited between metal/ceramic systems, such as between aluminum and silicon carbide.
Therefore there is need for a discontinuously reinforced metal matrix composite that has improved strength, stiffness and toughness and provides greater flexibility in processing. There is also a need for a processing method which allows for better processing control.